Fluid–structure interactions between elastic walls and shock trains in scramjet isolators under fluctuating backpressure

Shock train behavior in scramjet isolators is especially sensitive to boundary conditions. Although extensive studies have examined shock trains in rigid isolators, fluid–structure interactions (FSI) involving flexible walls receive comparatively little attention. This study employs a two-way coupled CFD/CSD approach to investigates the FSI between elastic walls at different cavity pressure and dynamic shock trains under varying backpressure fluctuation frequencies, with particular focus on structural response, flow features, and isolator performance. Under steady backpressure, elastic walls exhibit two primary deformation states: stable bending state at lower cavity pressure and flutter state at higher cavity pressure. When subjected to fluctuating backpressure, the elastic wall motion is governed by a competition between intrinsic flutter and forced vibration, resulting in distinct flow behaviors and performance. In the stable bending state, the wall predominantly undergoes forced vibration. As the backpressure frequency approaches resonance, wall vibration is amplified, shock train oscillations are suppressed, and the shock train shifts upstream. Beyond resonance, wall vibrations weaken and the shock train moves downstream. In the flutter state, the vibration mode transitions between flutter and forced vibration depending on the backpressure frequency. At lower frequency, flutter dominates with minor changes in the mean structural deformation and flow structure, although transient fluctuations are pronounced. Approaching resonance shifts the system toward forced vibration, significantly amplifying wall vibrations and altering the shock train location. Beyond resonance, the system reverts to flutter dominance, with limited variation in both mean and transient flow characteristics. Consequently, isolator performance is substantially affected by the mode transition. Moreover, compared to rigid-wall under fluctuating backpressure, elastic walls benefit downstream shock positioning but amplify unsteady flow behavior, particularly under flutter or near resonance, thereby posing challenges for isolator stability and downstream combustion.